Agua-suelo-clima

 

A new N index to assess nitrogen dynamics in potato (Solanum tuberosum L.) production systems of Bolivia

 

Un nuevo índice de nitrógeno para evaluar la dinámica de nitrógeno en sistemas de producción de papa (Solanum tuberosum L.) en Bolivia

 

Ana K. Saavedra¹, Jorge A. Delgado^2*, Ruben Botello¹, Pablo Mamani¹, Jeffrey Alwang³

 

¹ PROINPA, Cochabamba, Bolivia.

² USDA-ARS, Soil Plant Nutrient Research Unit, Fort Collins, CO 80526. ^* Author for correspondence (Jorge.Delgado@ars.usda.gov).

³ Virginia Polytechnic Institute and State University, Blacksburg, VA 24061.

 

Received: July, 2014.
Approved: September, 2014.

 

Abstract

Bolivia is South America's poorest country, with over 80 % of the rural population under the poverty line and agricultural productivity is closely inversely correlated with poverty in rural Bolivia. Potato (Solanum tuberosum L.) is one of the most important crops for food security in Bolivia, where it is grown with traditional methods, and national-level yields are low. Traditional management of potato involves use of organic amendments (animal manures) to supply N. Enhanced N management has the potential to raise potato yields and reduce rural poverty, yet there is a lack of information about improving nitrogen management practices in potato production areas where traditional systems predominate. A N index for traditional agricultural systems in Bolivia was developed and tested with metadata collected from two of the few studies available for high-altitude systems. The Bolivia N index was able to predict the nitrogen uptake for potato systems at these sites and uptake was correlated with yields (p≤0.001). Potato responded significantly to N inputs and to total N availability. The Bolivia N index can be used to assess management practices for traditional potato systems and can provide information to farmers and technicians, helping them improve N management to increase yields and food security in Bolivia.

Key words: Food security, high altitude cropping systems, nitrate leaching, Solanum tuberosum L., sustainable system.

 

Resumen

Bolivia es el país más pobre de Sudamérica, con más del 80 % de la población rural bajo la línea de pobreza y la productividad agrícola tiene una estrecha correlación inversa con la pobreza en el área rural de Bolivia. La papa (Solanum tuberosum L.) es uno de los cultivos más importantes para la seguridad alimentaria en Bolivia, donde se cultiva con métodos tradicionales, y los rendimientos nacionales son bajos. El manejo tradicional de la papa implica el uso de sustancias orgánicas (estiércol animal) para suministrar N. El uso de N mejorado tiene el potencial de aumentar los rendimientos de papa y reducir la pobreza rural, pero falta de información para mejorar las prácticas de empleo del nitrógeno en las áreas de producción de papa, donde predominan los sistemas tradicionales. Un índice de N de los sistemas agrícolas tradicionales en Bolivia se desarrolló y probó con metadatos recolectados de dos de los pocos estudios disponibles para sistemas de gran altitud. El índice de N de Bolivia pudo predecir la absorción de nitrógeno por los sistemas de papa en estos sitios y la absorción se correlacionó con el rendimiento (p≤0.001). La papa respondió significativamente a los aportes de N y a la disponibilidad de N total. El índice de N de Bolivia puede usarse para evaluar las prácticas de manejo de los sistemas tradicionales de papa y puede entregar información a los agricultores y técnicos, ayudándolos a mejorar el uso del N para aumentar los rendimientos y la seguridad alimentaria en Bolivia.

Palabras clave: Seguridad alimentaria, sistemas de cultivo de gran altitud, lixiviación de nitrato, Solanum tuberosa L., sistema sustentable.

 

INTRODUCTION

Potato (Solanum tuberosum L.), one of the most important crops in Bolivia, is planted in several locations ranging from the Andean regions to the Santa Cruz valleys to the Bolivian plateau. Rural livelihoods depend on potato yields, but production systems are vulnerable due to erosion on steeply sloped parcels and irregular rainfall and they are also not managed with sufficient nitrogen inputs to maximize the value of yields net of costs (Bottner et al., 2006). Agriculture in this region is semi-subsistent; potato provides as much as 50 % of food energy, and it can be a major source of household income (Alwang et al., 2013). Average yields in potato-producing areas near Tiraque, outside of Cochabamba, may be as high as 10 Mg ha^-1. The region is dominated by the use of the varieties Waycha and Desiree; other important varieties include Jaspe, Toralapa and Robusta. The traditional practice in these low input systems is to plant two potato crops per year where irrigation water is available, or a single crop in rainfed systems. In irrigated systems there is an early planting of misk'a from May to August. The intensive planting season, also known as the "jatun tarpuy", is conducted when the rainy season begins in October and November.

Typical rotations are potato — beans (Phaseolus vulgaris L.) or corn (Zea mays L.) — cereals — fallow (return to native grass system). In high-altitude systems, the main rotation is potato — cereals — fallow (return to native grass system). These rotations have short planting seasons for the crops that can last from one to three years followed by a fallow period (return to native grass system) that can last up to 20 years. During the fallow period, the native vegetation can regrow, improving the soil system and reducing the erosion (Bottner et al., 2006). In irrigated areas, the fallow period is significantly shortened or eliminated.

Cultivation of these vulnerable soils significantly impacts their fertility, reducing their productivity (Sarmiento and Bottner, 2002), which is aggravated by poor management practices where the crop residue is removed and used as forage, thus increasing the potential for erosion and preventing re-incorporation of residue nutrients into the soil. Some examples of good management practices for these systems include leaving crop residue in the field (to reduce erosion and preserve soil moisture), using minimum tillage, and leaving fields in native vegetation to be used for forage.

About 60 % of the 8.3 million people in Bolivia live in poverty, and for the rural areas there is 82 % of the population living below the poverty line (UNICEF, http://www.unicef.org/bolivia/resources_2332.htm). Low agricultural productivity, lack of infrastructure, and limited access to markets are correlated with high poverty in rural areas (UNICEF, http://www.unicef.org/bolivia/resources_2332.htm).

These issues are even more challenging in the context of climate change, where rainfall patterns are changing (e.g., shorter, yet more violent and irregular rainfall events). To optimize agricultural productivity and increase food security, conservation practices are needed, as well as improved management at the field level to increase adaptation to climate change (Delgado et al., 2011; Lal et al., 2011). Best nitrogen management is at the center of climate change adaptation and it is essential to increase productivity in vulnerable, low-input systems (Delgado et al., 2011; Lal et al., 2011).

Conservation agriculture has increased yields in low-input systems in Asia, Africa, and Latin America (Thiombiano and Meshack, 2009; Silici, 2010; Alwang et al., 2013). Conservation agriculture (FAO, 2009) can help increase yields and reduce vulnerability to climate-based fluctuations. Good nitrogen management can significantly contribute to increased yields while reducing nitrogen losses, which can improve water quality and raise the economic viability of fragile systems. Besides, the use of computer tools can contribute to improved nitrogen management (Delgado and Follett, 2010). There is a lack of information on improving yields in Bolivian cropping systems and using tools to improve nitrogen management to increase yields.

The available peer-reviewed literature has few, if any, papers on soil nitrogen dynamics, nitrogen uptake, and best management practices for low-input systems in Andean regions of Bolivia. Chemical fertilizers are used in some regions of Bolivia high plateau, but often the only fertilization is sheep and camelid manure. In areas such as the Tiraque region, use of purchased fertilizer has declined and chemical fertilizers were replaced by organic manure from chickens and cattle. The average amount of poultry manure applied in the Tiraque area is 8 Mg ha^-1. Decision makers in the Tiraque region lack information on nitrogen management, how to effectively estimate crop nitrogen requirements, and how to make nitrogen management decisions.

In the scientific literature reviewed no papers were found about the use of computer tools to improve nutrient management in these low-input cropping systems. New tools such as the Bolivia N index, which is bilingual (English and Spanish) and can be run in either English or IS units, have great potential to close the information gap. The N index is a simple but robust tool that can quickly assess the effect of nitrogen management practices on nitrogen uptake by crops, nitrogen use efficiency, and the risk of nitrogen losses to the environment (De Paz et al., 2009; Figueroa et al., 2011; Monar et al., 2013). It can be used as an extension tool to quickly provide information about nitrogen management. Additional information about the N index can be found in Delgado et al. (2006, 2008), Figueroa et al. (2011), and Monar et al. (2013).

The USAID Sustainable Agriculture and Natural Resource Management (SANREM) Collaborative Research Support Project (CRSP) has been working for more than eight years in the Tiraque region. SANREM CRSP is a collaboration among several entities from the USA and Bolivia, including Virginia Polytechnic Institute and State University (Virginia Tech), the USDA Agricultural Research Service (ARS) Soil-Plant-Nutrient-Research Unit (SPNRU), and the Fundación para la Promoción e Investigacion de Productos Andinos (Fundación PROINPA). As a part of the project's focus on conservation agriculture in these potato-producing areas, SANREM CRSP developed a new N index with a sustainability index for application in Ecuador and Bolivia (Monar et al., 2013). However, the new Bolivia N index has not been tested for this region.

Potato is highly sensitive to inputs; yields and even potato quality can be improved with good nitrogen management (Essah and Delgado, 2009). The Bolivia N index can quickly assess the effects of management practices on nitrogen dynamics, nitrogen uptake, and the potential risk of nitrogen losses to the environment. Conservationists, nutrient managers, and farmers can use it to improve management and increase yields.

This is the first use of the N index to evaluate application of nitrogen in low-input (low rate of organic N or inorganic N fertilizer) high-altitude, sustainable systems of Bolivia. The objective of this study was to use the few published data from two studies conducted with potato systems in Bolivia, to evaluate the performance of the new Bolivia N index in assessing nitrogen management.

 

MATERIALS AND METHODS

The new Bolivia N index was developed to assess the effects of management practices on cropping systems in Ecuador and Bolivia (Monar et al., 2013). To validate the study, the available scientific literature on potato systems in the Andean region of Bolivia and other Andean regions was reviewed and only two studies were found about potato systems and nitrogen management in Bolivia. These studies were conducted in Andean Bolivia in Patacamaya, La Paz state (Pestalozzi, 2000), and in Japo, Cochabamba (Couteaux et al., 2008). These sites have typical climate patterns for Bolivian high-altitude cropping systems, with a rainy summer season, November to March, and a cool winter dry season, April to October (Pestalozzi, 2000; Couteaux et al., 2008). Precipitation at Patacamaya was 334 and 316 mm for the first and second years of the study, respectively, the average annual mean temperature was 8.7 °C, the soil pH was 6.2 and the organic matter (OM) content was 0.6 %. In Japo, the annual precipitation was 400 mm, the average temperature was 6.6 °C, the soil pH was 4.9, and the soil OM ranged from 1.4 to 4.1 %.

Pestalozzi (2000) monitored 11 plots, six of which had native vegetation, and compared nitrogen dynamics during different fallow periods. Detailed descriptions of the nitrogen fertilizer and organic fertilizer or both from this study were entered into the N index. Four control plots that did not receive any type of nitrogen input were also established at these sites. The fallow periods monitored were in sites which were fallowed for 9, 19, and 20 years. Details of this study and that of Coteuax et al. (2008) are presented in Table 1. Couteaux et al. (2008) studied the dynamics of potato in a crop rotation of potato and grains and applied cattle manure during the two years of potato cropping (Table 1). The data in these studies about weather, precipitation, soil bulk density, soil organic matter, soil pH, yields, nitrogen fertilizer input, and organic inputs were entered into the Bolivia N index, which estimated the crop nitrogen uptake and nitrogen losses via different pathways. There is additional information about the N index, including its algorithms and capabilities, in Delgado et al. (2006, 2008), De Paz et al. (2009), Figueroa et al. (2011), and Monar et al. (2013). Version 4.4.2 of the N index can be downloaded from http://www.ars.usda.gov/npa/spnr/nitrogentools; the user manual for this tool is available at http://www.ars.usda.gov/SP2UserFiles/ad_hoc/54020700NitrogenTools/Nitrogen%20Index%204.4%20User%20Manual%20final.pdf.

Statistical analysis of the performance of the Bolivia N index was conducted by using the SAS regression analysis procedure (SAS, 1988) to determine the relationship between predicted and observed values.

 

RESULTS AND DISCUSSION

The nitrogen uptake of potato estimated by the Bolivia N index was correlated with the observed uptake across all these studies (Figure 1; p≤0.001) and the model was accurate in predicting actual crop uptake measured at the Cochabamba and Patacamaya sites. Mean potato tuber fresh yields were significantly correlated with observed N uptake in these representative sites (Figure 2; p≤0.001). In other words, for higher tuber yields, more nitrogen needs to be in the system to meet the crop's needs (Figure 2; p≤0.001). This response is similar to studies showing a yield response to nitrogen management and higher nitrogen inputs (Essah and Delgado, 2009).

The Bolivia N index calculates the nitrogen available in the system by estimating the nitrogen mineralization from the soil organic matter, as well as the nitrogen mineralization from the previous crop residue and from any organic amendments. The N index then sums the nitrogen mineralization estimates for these organic compartments with other nitrogen inputs such as atmospheric nitrogen, precipitation, and nitrogen fertilizer applied. Crop uptake is estimated by the N index.

These results of having more N removed by crop uptake than applied to the system, show that higher yields and higher N use efficiency can be obtained in a system that has low N losses to the environment. For example, the average N input from organic and inorganic nitrogen fertilizers at these 15 sites was 27.0 kg N ha^-1. However, the average N uptake from these sites was 49.9 kg N ha^-1, nearly twice as much. The average nitrogen cycling from the systems for these sites was estimated with the N Index to be 70.4 kg N ha^-1. These systems are low in nitrogen inputs, and the crop is mining N from the system due to mineralization of nitrogen from soil organic matter and from organic residues and manures that are being added to the system.

A system that is loaded with N will become saturated, and while this will meet the system's N needs, there will be large quantities of residual nitrate in the soil profile, increasing the potential for N losses and for low N use efficiency. However, this is not what is happening in these low-input systems where there are low applications of organic or inorganic N fertilizer, or both. The N fertilizer inputs are small, but large enough to increase the yields significantly above the control plots (plots with zero nitrogen fertilizer), with minimized nitrogen losses to the environment and the average nitrogen uptake from these sites was nearly twice as much as the N inputs.

This response is due mainly to the fact that nitrogen available from mineralization from crop residue incorporated into the soils has greater nitrogen use efficiency as compared to chemical fertilizer (Delgado et al., 2010). According to Delgado et al. (2010), nitrogen losses from crop residue were about three times lower than nitrogen losses from the readily available nitrogen fertilizer. For systems that are left fallow, the mineralization from the native grasses and nitrogen in the organic matter is increased when cultivated, providing significant nitrogen, with higher efficiency and a reduced need for nitrogen inputs (Delgado and Follett, 2002; Delgado et al., 2010).

The N index also provided an assessment of N losses to the environment. Nitrate leaching losses were minimal, from 0 to 5 kg NO₃-N ha^-1. This is in agreement with the results that the average N uptake from these sites was at nearly twice as much as the N input (equivalent to a high N use efficiency). This is also in agreement with results from studies about nitrate leaching assessments of dryland cropping systems such as those in the Great Plains of the USA (Westfall et al., 1996; Evans et al., 1994; Williams and Kissel, 1991). Precipitation rates for the Bolivian studies were 334 and 316 mm for Patacamaya during the first and second years of the study, respectively. The average annual precipitation for Cochabamba was 400 mm. The minimal nitrate leaching for these systems is in agreement with that from Williams and Kissel (1991), who report a threshold of 404 mm for minimal nitrate leaching in the Great Plains systems.

The N index's estimate of the N uptake of potato was correlated with the N index's estimate of the total N available in the system (Figure 3; p≤0.001). The potato crop will use a larger amount of N if more N is available. Similarly, there was a positive correlation between nitrogen available in the system and observed tuber yields (Figure 5; p≤0.001).

This evaluation of the Bolivia N index showed that there is potential to assess the N available in these systems, and that there is a correlation between potato yields and N uptake. The assessment suggests a strong correlation between yield and the total nitrogen available in the system. The Bolivia N index can be used to quickly assess the potential nitrogen available from soil organic matter, organic manures, and fertilizer, avoiding the need for costly and timeconsuming measurements.

Figueroa et al. (2011) used the Mexican N index to evaluate the nitrogen dynamics of forage systems in Mexico, which often have large quantities of organic manure added to them. The index was able to predict soil nitrogen dynamics by accurately predicting the nitrogen uptake for the forage system and residual nitrate in the soil profile (p≤0.05). The N index was also used to predict nitrate leaching losses from irrigated systems of a Mediterranean region in Spain with a large amount of nitrogen input into the systems (De Paz et al., 2009). This is the first time the index was used to evaluate low-input sustainable systems, and the results suggest that the index can be used in Bolivia and other Andean regions (Monar et al., 2013) to evaluate N dynamics.

As Bolivia is the poorest country in South America, and poverty and agricultural production are related to one another, enhanced agricultural productivity has strong potential to benefit rural locations, where poverty is highest. This paper demonstrates that enhanced nitrogen management and increases in nitrogen inputs into potato production systems will significantly increase yields and improve the standard of living of families across rural Bolivia.

This is the first time that the N index was used to evaluate potato systems from low-input agricultural systems in the Andean region of Bolivia. The N index can be used to quickly conduct an analysis across the soils of this region and help users determine how nitrogen needs to be applied to optimize yields. The index is available as a mobile application for smartphone and tablet systems; the mobile application can be used right in the field and can email the results of a run to any desktop or laptop computer (Delgado et al., 2013). As a result of this easy access, personnel on the ground can use the index to assess the risk of nitrogen losses from the system and increase yields (Figures 1, 2, 3, 4, 5).

There is potential to increase nitrogen applications in these systems without increasing nitrogen losses to the environment, as long as conservation agriculture is also applied to avoid potentially increasing erosion. Future plans include testing a sustainability index that has also been added to the Bolivia N index (Monar et al., 2013). The use of quick and simple assessment index such as the Bolivia N index can help users identify management alternatives.

 

CONCLUSIONS

The results suggest that the Bolivia N index can accurately assess N dynamics and uptake in vulnerable, high-elevation Andean potato cropping systems. The study also identified clear potential gains from improved N management. The N index can be used to estimate the N uptake needs of the potato systems. This index can help evaluate nitrogen management practices, nitrogen losses from the system, and uptake of nitrogen fertilizer applications.

 

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